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With all the cold weather, I posed the following question to my siblings:

What’s the coldest temperature air that a human can safely breathe?

Mind you, my sister spent a year at the South Pole, so she knows about cold. If you inhale cold air too fast, your lungs get frostbite– which can kill you. It really does happen when people exercise in extreme cold. (And for those of you in southern California, extreme cold starts somewhere below -10 F.) So I’m assuming you breathe slowly, giving the air plenty of time to warm up before hitting the lungs. I’m also assuming you’re wearing extremely warm clothing, though little more than a scarf over your nose and mouth. No special breathing apparatus.

Here’s what we came up with: Oxygen condenses to a liquid at -297 F. Which is also to say, liquid oxygen boils at -297 F. So anything beyond that and you’re drinking oxygen rather than breathing. At that point, the carbon dioxide has long since solidified and the nitrogen has liquefied, so presumably you can get pure oxygen– so you don’t have to breathe quite so much. Remarkably, the amount of energy needed to boil oxygen is about the same as the amount needed to bring it to body temperature. Assuming you need 6 lbs of oxygen a day, it’s about 5 calories per hour to boil oxygen, plus another 5 calories to bring it to room temperature. If you could figure out a technique for heating it up without getting frostbite, you could actually survive on liquid oxygen. You wouldn’t even lose much weight.

Of course, that’s a big if. You also need to get plenty of water, since there won’t be any in the air, and dry cells are more prone to frostbite. (Water stores a lot of heat.)

What’s remarkable is that, under the right circumstances, a mammal could evolve to live in extreme cold by drinking liquid oxygen. I imagine something like a deer with a very specialized trunk for transferring heat to the oxygen. Perhaps it would even have sacks for storing liquid oxygen. It would have extremely high caloric requirements, not because of the oxygen, but because it would presumably need to melt its water and keep its body warm. (There are actually herd animals in Asia that get all their water from frost.)